1. Field of the Invention
This invention relates generally to oscillators and more specifically to oscillators with variable reference.
2. Description of the Background Art
For signal processing circuitry to operate linearly and optimally process an analog signal, the analog signal should be within a limited range of amplitude. For example, if an input signal to the signal processing circuitry is too low in amplitude, noise that is present in the signal processing circuitry represents a relatively large part of the input signal. The input signal that is too low in amplitude results in a poor signal-to-noise (“S/N”) ratio in an output of the signal processing circuitry. Conversely, if the input signal to the signal processing circuitry is too high in amplitude, then the signal processing circuitry can operate nonlinearly, resulting in distortion or “clipping” in the output.
FIG. 1 illustrates circuitry including automatic gain control (“AGC”) in the prior art. AGC 110 detects an output amplitude at an output 130 of the signal processing circuitry 100 to control the gain of a gain adjustment amplifier 120 in the signal processing circuitry 100. If the output amplitude at the output 130 is too low in amplitude, then the AGC 110 increases the gain of the gain adjustment amplifier 120. Conversely, if the output amplitude at the output 130 is too high in amplitude, then the AGC 110 decreases the gain of the gain adjustment amplifier 120.
One problem with the AGC approach of FIG. 1 is that extraneous components are required in the signal processing circuitry 100 to support the AGC function, but are not otherwise needed. For example, the gain adjustment amplifier 120 of FIG. 1 is necessary for AGC, but does not perform any signal processing function in the signal processing circuitry 100. Another problem is that extraneous components in the signal processing circuitry 100 contribute undesired noise and/or distortion in the output of the signal processing circuitry 100.
FIG. 2 illustrates a circuit 200 to control an amplitude of an oscillation signal in the prior art. The circuit 200 limits an amplitude of the oscillation signal at an input 210 of the signal processing circuitry 240 to within a predetermined range. An oscillator 222 generates the oscillation signal that a buffer 215 amplifies to create the input 210 of signal processing circuitry 240. A peak detector 230 compares the input amplitude at the input 210 of the signal processing circuitry 240 to a predetermined range 235 and generates a control signal 250 to control the amplitude of the oscillator 222. The amplitude of the oscillation signal is adjusted based upon a desired input amplitude at the input 210 of the signal processing circuitry 240.
The approach of FIG. 2 excites oscillation in the oscillator 222, minimizes phase noise or jitter in the oscillation signal at the input 210 or input 260, or reduces the power consumed by the oscillator 222. However, one problem with the circuitry 200 of FIG. 2 is that the amplitude of the oscillation signal generated by the oscillator 222 is based upon only a predetermined range of amplitude, and not upon the actual amplitude of signals at some node in the signal processing circuitry 240 or another component following or “downstream” of the oscillator 222 such as the comparator 270. For example, the amplitude of the signal at the input 260 to the comparator 270 may be too high for the comparator 270 to operate optimally if the signal processing circuitry 240 experiences an increase in its gain characteristics due to process variations, voltage fluctuations in a power supply (not shown) powering the signal processing circuitry 240, and/or a change in operating temperature of the signal processing circuitry 240.
Therefore, a need exists to address the aforementioned deficiencies and inadequacies.